1. Field of the Invention
The present invention generally relates to a disk drive, and more particularly to a disk drive which minimizes the track misregistration (TMR) error produced by theta-dynamics by a rotational vibration velocity-based sensor.
2. Description of the Related Art
The sustained data rate of a hard disk drive (HDD) is typically degraded in the presence of rotational vibration of a computer mounting system. Rotational vibration (RV) can result from random seek activity among a cluster of HDDs, and the customers have become concerned over the potential for performance degradation.
At high tracks per inch (TPI), the in-plane rotational vibration (theta coordinate) of a disk drive, referred to as “theta-dynamics”, directly impacts the head positioning accuracy. A solution to this vibration challenge can be developed along several disciplines, ranging from novel mount systems to sophisticated sensors and servo algorithms. However, cost effective sensing of the rotational vibration (RV) velocity or acceleration is a problem, which the conventional structures have not found a solution to yet.
Rigid body motion of the base plate of a disk drive can take place along three linear (X, Y, Z) and three angular (phi, psi, theta) coordinates.
The present generation of 1.0″, 2.5″ and 3.5″ hard disk drives (HDDs) are designed to operate in portable and desk-top/server environments, respectively. To reduce cost and weight of a computer system, manufacturers typically fabricate the HDD mounting frame utilizing thin structural members. Therefore, a computer frame is a compliant object which makes it susceptible to vibration. Such a mounting configuration makes a disk drive vulnerable to vibration excited by internal or external sources. An HDD with a rotary actuator system is highly sensitive to in-plane rotational vibration (RV) of its base plate.
Additionally, an HDD includes a head positioning servo system which performs three critical tasks.
First, the servo system moves the head to the vicinity of a target in a minimum time using a velocity servo under seek mode. Next, it positions the head on the target track with minimum settle-out time using a position controller without an integrating term (e.g., capability) in it. Finally, the servo system enters the track follow mode with a proportional-integral-derivative type (PID) position controller.
However, during the seek mode, maximum rotational acceleration torque followed by a deceleration torque is imparted by a voice coil motor (VCM)-based actuator. The corresponding reaction torque on the base-plate causes transient rotational vibration that can be detrimental to the positioning accuracy of the read/write heads. However, the presence of random vibration impacts the track following precision (and, slightly less, the settle-out performance).
Prior to the present invention, there has been no adequate addressing of the problem of random vibration as it critically affects the track following precision of an HDD actuator system.
Present 3.5″ disk drives have reached 40 kTPI, and after year 2001 it is expected to grow beyond 50 kTPI. A major obstacle to raising the track density is inadequate head positioning accuracy in the presence of vibration disturbances. Due to exponential growth in TPI, positioning the read/write elements over a track has become a major challenge. Conventional servo control system requires continuous innovations to perform well under increasingly difficult operating conditions.
The mechanical components such as spindle motor assemblies are not perfectly mass-balanced, and during operation they produce harmonic vibration. Harmonic vibration excitation produces both a linear and a rotational oscillatory motion of the entire HDD system. When not compensated, a track following error of 15% of the track pitch can be detrimental to a disk drive's “soft” and “hard” error rate performance. The positioning error due to this internally produced periodic vibration can be solved using a servo method disclosed in U.S. Pat. No. 5,608,586, incorporated herein by reference.
By using special shock and vibration isolation mount design, the rotational oscillatory components due to internal spindle forcing can be minimized as taught by U.S. Pat. No. 5,400,196, incorporated herein by reference. However, a mount design optimized to decouple internal spindle vibration as disclosed by U.S. Pat. No. 5,400,196, remains susceptible to external input vibration. By deploying the isolation mounts along a polygon satisfying a particular set of criteria defined by Japanese Patent No. 2,565,637, the external vibration inputs generating rotational vibration on an HDD can be minimized.
In U.S. Pat. No. 6,122,139, also incorporated herein by reference, a method to neutralize the reaction by generating a counter torque using a secondary actuator is proposed. An HDD with a novel sensing and control solution could provide an enhancement to the problem of random vibration.
As shown in FIGS. 1A–1C, U.S. Pat. No. 5,721,457, incorporated herein by reference, shows a dual PZT configuration 101, 102 in a disk drive where the mass and inertia of the disk drive is exploited as the seismic body to measure angular and linear acceleration with substantial sensitivity.
That is, FIG. 1(a) illustrates a head disk assembly 100, FIG. 1(b) illustrates in greater detail the piezoelectric strain sensor 101, 102 for measuring acceleration, and FIG. 1(c) illustrates the head disk assembly 100 on a userframe 104 undergoing shock and vibration, with the dual PZTs 101, 102 providing an angular and linear acceleration inputs to a component 105, thereby resulting in a write inhibit signal being issued.
A key challenge in the use of PZTs is that they are sensitive to strain along multiple axes, and therefore they respond to vibration inputs in addition to the theta-dynamics.
To produce high fidelity signals in the range of 100–1000 Hz, the size of a PZT configuration must be large and such a design is not compatible with the electrical card height and manufacturing requirements in a disk drive. On the other hand, reducing the PZT volume produces poor signal quality (i.e., particularly the signal drift in the low frequency range (˜100 Hz) is not easily stabilized).
The measurement-based experience of the present inventors is that the signal stability and noise are key problems in employing a compact PZT configuration. Sudden drift in a PZT signal can cause undesirable write-abort condition. Use of dual PZTs further complicates the problem of matching the individual PZT gain and thermal sensitivity. By providing novel mechanical structures, the sensitivity of a PZT can be enhanced along the desired direction and minimized along the remaining directions. However, the stringent decoupling requirements of dynamics makes the cost of a dual PZT sensor cost prohibitive for a disk drive application.
By deploying dual PZT sensors 201, 202, as shown in FIG. 2, and a signal conditioning algorithm, a conventional system 200 (e.g., see A. Jinzenji et al. “Acceleration feedforward control against rotational disturbance in hard disk drives,” APMRC-Nov. 6–8 2000, TA6-01–TA6-02; U.S. Pat. No. 5,426,545 to Sidman et al., incorporated herein by reference) demonstrates a feedforward solution to random vibration. PZT sensors 201, 202 by themselves do not produce high quality output without additional innovation. Figure also illustrates a feedforward compensator 203 and a conventional servo 204.
An alternative approach uses a capacitive sensing micromechanical device (e.g., see C. Hernden, “Vibration cancellation using rotational accelerometer feedforward in HDDs,” Data Storage, November, 2000, pp.22–28), which attempts to produce a quality theta-acceleration sensor. However, sensor size, bandwidth and cost are considered to be limitations of a microelectromechanical sensor (MEMS).
Thus, the conventional sensors have been unable to deal adequately with the problem of random vibration, as it critically affects the track following precision of an HDD actuator system, and no sensor has been produced with is cost effective and effective for sensing rotational vibration (RV) velocity or acceleration.